The present invention relates to the detection of damage to a gas turbine engine from foreign object ingestion and to accommodation of such damage.
Many gas turbine engines include a low pressure compressor, a high pressure compressor, a combustor and at least one turbine. The low pressure compressor has an intake through which air at ambient pressure is drawn. The pressure of the air, a working fluid, is increased as it flows through this compressor. At least a portion of the working fluid is passed from the low pressure compressor to an adjacent high pressure compressor aligned therewith. This high pressure compressor further increases the pressure of working fluid flowing through it. Next, high pressure working fluid exiting the high pressure compressor is mixed with fuel and ignited by one or more combustors creating rapidly expanding combustion gases which drive one or more turbines. A turbine extracts power from the combustion gases before these gases exit a rear nozzle as exhaust. In some configurations, a thrust vector is created in the direction opposite the exiting exhaust gases.
One common gas turbine engine configuration has coaxial twin spools. This configuration has a low pressure spool including a low pressure turbine connected by a shaft to a low pressure turbine, and a high pressure spool or core including a high pressure turbine connected by a separate coaxial shaft to a high pressure compressor. Combustors are situated between the high pressure compressor and high pressure turbine of the core, and the core is situated between the low pressure compressor and low pressure turbine along a common working fluid path so that the compressors and turbines are adjacent one another. For a twin spool turbofan engine with a high bypass design, a large portion of the working fluid passing through the intake is bypassed by a stage of the low pressure compressor, a fan, through a bypass duct. The bypassed air typically blends with the exhaust gases exiting the exhaust nozzle. As a consequence, greater efficiency and reduced engine noise result.
Unfortunately, the fan is susceptible to damage when birds, ice, or other foreign objects are ingested into it. The introduction of foreign objects into the air intake can have catastrophic consequences and it has been known that, for example, metal objects have produced serious structural faults in engines particularly during take-off and landing. Such catastrophic results may lead to a stall event within the engine. U.S. Pat. No. 3,852,958 to Adams et al., U.S. Pat. No. 3,867,717 to Moehring et al., and U.S. Pat. No. 4,603,546 to Collins disclose various schemes to detect stalls resulting from ingestion of a foreign object. However, not all ingestion events result in a stall. Instead, the damage may only cause thrust loss. Indeed, it is desirable to detect foreign object damage which results in a 25% thrust loss irrespective of a stall condition and to accommodate that loss by providing at least 75% of the thrust available prior to damage. Often, when a medium sized object such as a bird or ice is ingested into a high bypass engine, only fan degradation results. This result may be due to centrifugal forces imparted to an object that encounters the fan which drive it outward so that it passes through the bypass duct and avoids entry into the remainder of the engine. Commonly, the damage curls or otherwise deforms fan blades, and so reduces the ability of the fan to pump air for a given rotational speed. In this instance, an unexpected change of relative rotational speed between the fan and other rotating members is likely, but not a stall. Consequently, what is needed is a way to detect fan damage based on relative speed and to accommodate that condition by recovering thrust to a predictable level.
With the advent of digital avionics control systems, more sophisticated approaches to fan damage detection have arisen. U.S. Pat. Nos. 4,959,955 and 5,072,580 both to Patterson et al. are one such approach. These patents rely on measurement of the Engine Pressure Ratio (EPR) which is the ratio of pressure leaving the gas turbine to the pressure entering the compressor. This detection system is based on the unique relationship between engine air flow, exhaust nozzle area and engine pressure ratio for a turbofan that is undamaged. Fan damage for an EPR controlled system results in reduced fan speed and air movement capability for a given thrust setting. Specifically, detection is possible by comparing the actual EPR for the damaged fan to the predetermined EPR, air flow, and exhaust nozzle area relationship for the undamaged fan. This EPR error based detection scheme integrates well with an overall EPR thrust control system which uses EPR as a primary feedback signal, and already includes a number of pressure sensors. Once fan damage is detected with this system, adjustment of the exit area of a variable exit nozzle is made to correct for the EPR error measured, and the EPR thrust control system is abandoned in favor of an unspecified base mode control system.
Unlike an EPR based thrust control system of the Patterson et al. patents, other thrust control systems exist which do not rely on pressure measurements along the gas turbine engine. For example, one scheme measures the fan speed as the primary feedback signal. This thrust control employs a speed sensor in lieu of the pressure sensors used in an EPR based thrust control system. Furthermore, not all gas turbine engines have a variable exit nozzle which may be adjusted in response to detected damage. Thus, a need still exists for a fan damage detection and recovery system that readily integrates with a fan speed thrust control system and provides for predictable thrust recovery without resort to a variable exit nozzle adjustment.